1. Field of the Invention
The present invention relates to a method and apparatus for purifying silicon to such an extent as to be suitable for solar cells.
2. Description of the Prior Art
High-purity silicon for solar cells should have a resistivity greater than, say, 0.1 .OMEGA..cm. Therefore, it is necessary that the content of impurities in such high-purity silicon be of the order of ppm. Much has been studied to achieve this goal; however, boron and carbon still remain difficult to remove.
One known method for removing boron is by melting silicon under a high-temperature plasma generated by radio-frequency excitation. (See Japanese Patent Laid-open No. 218506/1988.) According to the disclosure, this method consists of a first step of treating silicon with a (mixed) gas composed of 1-100% hydrogen and 99-0% argon, and a second step of treating silicon with a plasma generated in a mixed gas composed of 0.005-0.05% oxygen and 1-99.995% hydrogen. This method has some disadvantages, including:
(a) Wastefulness resulting from the use of a thermally inefficient plasma to both melt and purify silicon. PA0 (b) Low productivity inherent in a plasma which merely melts a comparatively small limited are of silicon (which does not warrant the mass production of solar cells). PA0 (c) Restrictions on increasing the oxygen concentration in plasma gas which are inevitable to reduce the loss (by scattering and evaporation) of silicon that occurs due to local excessive heating during purification.
As to the removal of carbon, there is disclosed in Japanese Patent Laid-open No. 292613/1987 a method which consists of blowing argon gas (under reduced pressure) over molten silicon in a silica crucible in such a manner as to stir the molten silicon. This method is slow in decarburization and poor in productivity.